How is this possable?!!!

[mickytuff]

I tested some rcbo's the other day, they made by EATON i think, and when tested at the device in the board they wouldnt trip on x1 or x5, but when tested on their circuit ie the socket or light fitting, they trip as they should, very strange and dont know why?????, please can anybody advise why this happened, i tested bout 7-8 rcbo's and they all did the same thing, trip at the point they were serving but not at the board.....................................

 

[Top Cat]

Can't see why that would happen, but saying that i don't test at the board.

Are you sure you were keeping prongs of meter in contact all the time with RCBO because if you went open circuit it would not trip?

 

[RISElectrical]

Sure you were on the outgoing side I've never experienced this before however as said I rarely if ever test at db

 

[mickytuff]

no the prongs were held on tite, just cant think of a reason why, tested the meter on other rcbo's and rcd's afterwards (different makes though) and all tested fine, even at the board..................................

 

[mickytuff]

the db was a 3 phase board with single phase rcbos, much quicker to test at the board thats why i normally do it.

 

[Amp David]

I reckon you had the probes on the supply side of said RCD LOL

Eaton to me are nothing but reliable!

 

[ringer]

With RCDs I always seem to go to the supply side instead of the load side to test, then wonder why it won't trip. Doh! One of these days I'll learn to do it right first time!
But with RCBOs it is pretty obvious which side is which, so less easy to put the probes on the wrong side.
Not come across any myself, but I'm sure I read about some RCDs needing some kind of load for testing, even if it is just a short length of cable? Could that be the case with these RCBOs?

 

[Engineer54]

All of the manufactures advise testing should be conducted at the load side of the RCD device itself, although some (can't remember which) advise a testing from a short leg of wire rather than directly on the load terminals....
None advise proof testing at socket outlets etc.... You are after all, ...testing the RCD unit itself, not the RCD and the circuit it's connected too!!!

 

[Darkwood]

common issue is a coating on the grub screws --- anti corrosion will render the test void, if you put the probes within the clamp of rcbo and tightened down im pretty sure it would test ok, and unlike other responses im pretty sure if you are doing these tests you know the basics of the supply side and load.
On many occasions ive done voltage check via grub screw head and got strange reading due to this.

 

[MarkieSparkie]

As E54 has highlighted there was a post the forum last year where a Sparky had encountered the same problem with MEM (part of Eaton) RCBOs. The Sparky contacted the manufacturer and was told that their RCBOs require a minimum final circuit length of 1m when conducting RCD testing. He was totally amazed by this revelation, particularly because the instruction leaflet supplied with RCBO made no mention of this fact and posted to warn forum members.

 

[Engineer54]

As E54 has highlighted there was a post the forum last year where a Sparky had encountered the same problem with MEM (part of Eaton) RCBO. The Sparky contacted the manufacturer and was told that their RCBOs require a minimum final circuit length of 1m when conducting RCD testing. He was totally amazed by this revelation, particularly because the instruction leaflet supplied with RCBO made no mention of this fact and posted to warn forum members.

I don't think it was 1m, lol!!...more like a few inches!! For some reason (maybe darkwoods explanation above) these RCD devices didn't always like being tested directly off the units terminal screws.....

 

[Adam W]

I was taught to test at a socket or accessory (eg light fitting).
This makes sense to me because it tests as the circuit is going to be used, rather than doing it at the CU where you could be effectively bypassing the functional earth wire.
It doesn't seem like any extra work because you either use the 3 leads as you would in the CU or just plug the adaptor lead into a socket.

 

[oba]

If it was the mem/eaton pod you can't test these at the db as the live terminal screw is part of the mcb.
The rcd pod can be snapped off allowing you to change mcb rating, this leaves you with a pod with a neutral terminal a neutral lead and a functional earth lead - no live rcd terminal just a hole to access the mcb live terminal

 

[londonlec]

As others have said MEM can't be tested at the device, I also found out the hard way!

 

[Engineer54]

I was taught to test at a socket or accessory (eg light fitting).
This makes sense to me because it tests as the circuit is going to be used, rather than doing it at the CU where you could be effectively bypassing the functional earth wire.
It doesn't seem like any extra work because you either use the 3 leads as you would in the CU or just plug the adaptor lead into a socket.

For device ''Proof'' testing All the manufacturers advise to test at the device load terminals and ''Not'' at utilisation points on the circuit it is connected too. The exception was, that one manufacturer suggested that short bits of wire be connected to the load terminals and tested from these. As stated before your testing the RCD device, NOT the circuit!!

Nothing to stop you testing from say socket outlets as confirmation of tripping, but not proof testing of the devices, as the circuit (that should have been previously electrically tested anyway) would be then be forming part of the tests current flow.... Why would you be effectively by-passing the functional earth, testing at the CU/DB??